version bump + minor test changes according to @parthea review

Author: tworec

doc/source/io.rst

@@ -4678,7 +4678,7 @@ are not supported.
 Integer and boolean ``NA`` handling
 +++++++++++++++++++++++++++++++++++
 
-.. versionadded:: 0.19
+.. versionadded:: 0.20
 
 Since all columns in BigQuery queries are nullable, and NumPy lacks of ``NA``
 support for integer and boolean types, this module will store ``INTEGER`` or